MEAK-7 Forms an Alternative mTOR Complex with DNA-PKcs in Human Cancer

Connor Ray,Fatima Sarah Haidar,Daniela Baccelli Mendonça,Joe Truong Nguyen,Jin Koo Kim,Paul H. Krebsbach,Alexandra Lucienne Fox

doi:10.1016/j.isci.2019.06.029

Abstract

SummaryMTOR associated protein, eak-7 homolog (mEAK-7), activates mechanistic target of rapamycin (mTOR) signaling in human cells through an alternative mTOR complex to regulate S6K2 and 4E-BP1. However, the role of mEAK-7 in human cancer has not yet been identified. We demonstrate that mEAK-7 and mTOR signaling are strongly elevated in tumor and metastatic lymph nodes of patients with non-small-cell lung carcinoma compared with those of patients with normal lung or lymph tissue. Cancer stem cells, CD44+/CD90+ cells, yield elevated mEAK-7 and activated mTOR signaling. mEAK-7 is required for clonogenic potential and spheroid formation. mEAK-7 associates with DNA-dependent protein kinase catalytic subunit isoform 1 (DNA-PKcs), and this interaction is increased in response to X-ray irradiation to regulate S6K2 signaling. DNA-PKcs pharmacologic inhibition or genetic knockout reduced S6K2, mEAK-7, and mTOR binding with DNA-PKcs, resulting in loss of S6K2 activity and mTOR signaling. Therefore, mEAK-7 forms an alternative mTOR complex with DNA-PKcs to regulate S6K2 in human cancer cells.

Highlights

Aberrant mechanistic target of rapamycin signaling has been observed in many types of human cancer (Saxton and Sabatini, 2017)
RESULTS mEAK-7 Protein Levels Are Elevated in Metastatic Human Non-Small Cell Lung Carcinoma Lymph Nodes mEAK-7 protein levels appear to be disproportionately high in human cancer cell lines when compared with non-cancerous cells (Nguyen et al, 2018), this limited observation does not exclude the possibility that mEAK-7 is present in healthy human tissues, because mechanistic target of rapamycin (mTOR) expression is found in many tissue types (Kim et al, 2002)
DNAPKcs inhibition resulted in a substantial decrease in S6K2/mTOR/DNA-PKcs binding, as well as S6K2 functional activity (Figure 5I). These results suggest that mEAK-7 structurally links DNA-PKcs and S6K2 to mTOR signaling

Summary

Introduction

Aberrant mechanistic target of rapamycin (mTOR) signaling has been observed in many types of human cancer (Saxton and Sabatini, 2017). Canonical models of mTOR complex 1 (mTORC1), the traditional S6K regulators, and mTORC2 may not exist in all cell types As examples of this phenomena, an mTOR complex that involves GIT1, which is distinct from mTORC1 and mTORC2, has been identified in astrocytes (Smithson and Gutmann, 2016), and ETS Variant 7 is capable of binding to mTOR and sustaining mTOR signaling in the presence of rapamycin (Harwood et al, 2018). These pivotal findings disrupt conventional ideas regarding the existence of only two mTOR complexes and suggest the possibility of other, unidentified mTOR complexes

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